This invention relates generally to high intensity discharge lamps wherein the arc discharge is generated by a solenoidal electric field and more particularly to use of a novel combination of fill materials in the arc tube component of such lamp to generate white color lamp emission at improved efficacy and color rendering indices.
The lamps described in the present invention are part of the class referred to as high intensity discharge lamps (HID) because in their basic operation a medium to high pressure gas is caused to emit visible wavelength radiation upon excitation typically caused by passage of current through an ionizable gas such as mercury or sodium vapor. The original class of such HID lamps was that in which the discharge current was caused to flow between a pair of electrodes. Since the electrode members in such electroded HID lamps were prone to vigorous attack by the arc tube fill materials, causing early lamp failure, the more recently developed solenoidal electric field lamps of this type have been proposed to broaden the choice of arc tube materials through elimination of the electrode component. Such more recently developed solenoidal electric field lamps are described in U.S. Pat. Nos. 4,017,764; 4,180,763; and 4,591,759, all assigned to the assignee of the present invention, and generate a plasma arc in the arc tube component during lamp operation, all in a previously known manner.
Such electrodeless HID lamps suffer from a number of problems, however, which primarily cause these lamps to operate less efficiently than other type lamp designs. Lamp efficiency or "efficacy", as used in the present application, means luminous efficacy as measured in conventional terms of lumens per watt. A different type problem experienced with electrodeless lamps is that they exhibit lower than acceptable color rendering capability in order to be employed for general purpose illumination. More particularly, general purpose illumination requires that objects illuminated by a particular light source display much the same color as when illuminated by natural sunlight. Such requirement is measured by known standards such as the C.I.E. color rendering index values (CRI), and CRI values of 50 or greater are deemed essential for commercial acceptability of lamps in most general lighting applications. A still further requirement for commercially acceptable general purpose illumination is the white color temperature provided with such lamp, which is fixed at about 3000.degree. K. for the warm white lamp, about 3500.degree. K. for the standard white lamp and about 4200.degree. K. for the cool white lamp, as measured by the C.I.E. chromaticity x and y values. It is a further generally recognized principle that increasing efficacy for such type discharge lamps impairs the lamp color rendering capabilities. Thus, while the prior art electrodeless lamps partially meet the foregoing merit criteria as a result of utilizing some of the same arc tube fill materials that are employed in the present invention, it has not yet been recognized that a particular combination of all such arc tube materials is needed to achieve color improvement without adverse impact on efficacy in such lamps.
Accordingly, it is a principal object of the present invention to provide solenoidal electric field lamps demonstrating improvement in both efficacy and color rendition at white color temperatures.
Another object of the present invention is to provide a particular design for solenoidal electric field lamps which optimizes performance achieved with present arc tube fill materials.
Still another important object of the present invention is to operatively associate the arc tube fill materials for a solenoidal electric field lamp with the lamp structural configuration in a manner which optimizes the lamp performance.
The foregoing and other aspects of the present invention together with the features and advantages thereof, will become apparent from the following detailed description, when read in connection with the accompanying drawings.